Definition of Uterus

The hollow muscular organ that supports and contains a pregnancy.

Eight ligaments—one anterior, one posterior, two round, two broad (also called lateral), and two uterosacral—suspend the pear-shaped uterus in the lower central abdomen (pelvis), with the narrow end of the uterus angled somewhat downward.

This suspension system allows the uterus, also called the womb, to expand during pregnancy. The fallopian tubes join the uterus, one on each side of the wide upper section called the fundus. The fundus angles forward such that the uterus lies above the urinary bladder.

The lower section of the uterus is the cervix, a thick neck of muscular tissue that joins the uterus with the vagina, the passage to the outside of the body. In its nonpregnant state the uterus is about three inches long and an inch thick; in pregnancy the uterus expands to become nearly 10 times as large as its nonpregnant size.

Within four to six weeks after childbirth the uterus returns to nearly its prepregnant size. The uterus has two layers of structure: the outer myometrium and the inner endometrium.

The surgical operation to remove the uterus is hysterectomy, which may be treatment for endometrial cancer, severe endometriosis or uterine fibroids, or dysfunctional uterine bleeding (dub).

The Myometrium

The myometrium is three layers of strong, smooth (involuntary) muscle. The fibers of the innermost layer form two circular patterns that emanate from the fallopian tubes and extend to the cervix. The fibers of the middle layer occur in random patterns that run lengthwise, widthwise, and diagonally.

These fibers primarily support the network of blood vessels that nourish the myometrium. The outermost layer’s fibers wrap diagonally (transversely) around the uterus.

The myometrium grows during pregnancy to accommodate the growing and enlarging fetus. Through mechanisms doctors do not fully understand, the myometrium begins rhythmic and increasingly intense waves of contractions, synchronized across the three layers of muscle, that ultimately result in childbirth.

The contractions stretch and thin (efface) the cervix and then push the fetus through the cervix, into the vagina, and out of the body.

The Endometrium

The inner structure of the uterus is the endometrium, a membranous tissue that contains abundant blood vessels and glands. The endometrium responds to the monthly hormonal cycle of estrogen and progesterone peaks and troughs, thickening when blood levels of estrogens rise—a preparation for pregnancy—and sloughing when estrogen drops and progesterone rises-menstruation.

When these hormonal cycles cease with menopause, the endometrium enters a state of atrophy, in which it remains for the rest of the woman’s life.

AdenomyosisDysfunctional uterine bleeding (dub)
Endometrial cancerEndometrial hyperplasia
EndometriosisPelvic inflammatory disease (pid)
PregnancyUterine fibroids

For further discussion of the uterus within the context of the structures and functions of reproduction and sexuality, please see the overview section “The Reproductive System.”


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